Polarization Degree and Direction Angle Effects on a CdZnTe Focal Plane Performance

To date in astrophysics, X- and gamma-ray source emissions have been studied almost exclusively through imaging techniques, spectral- and timing-variability analysis. However, often two or more distinct models potentially describe the same observations of a particular celestial-radiation source. By measuring the angle and degree of linear polarization of the detected radiation, it is increased by two the number of observational parameters thereby allowing a better discrimination between different models. Therefore, future $\gamma$ -ray-telescope instrumentation should include the capability to perform highly sensitive polarimetric measurements. This idea has recently become even more important from the perspective of new very sensitive hard X- and soft gamma-ray focusing missions based on Laue lenses such as the Gamma-Ray Imager and DUAL missions, submitted to ESA Cosmic Vision calls. In order to optimize the design of a gamma-ray telescope focal plane, a CdZnTe detector prototype has been tested at the European Synchrotron Radiation Facility under a ${\sim}100\hbox {%}$ polarized gamma-ray beam. The tested detector was a 5 mm thick CdZnTe array with segmented anodes into 16 $,\times,$ 16 pixels of 2.5 $,\times,$ 2.5 mm $^{2}$ . The aim of these tests was to study the polarimetric performance of this prototype as a function of the polarization state of the synchrotron beam. In particular, we determined the sensitivity of the detector to the level of the beam polarization as well as its potential to determine the polarization angle orientation by performing measurements at different polarization direction angles. In both tests the CdZnTe pro- otype showed a good linearity in its response and good sensitivity to polarization levels down to 50% and a polarization angular resolution better than 5 $^{\circ}$ .